Ionic Bonding (6) More on Lattice Energy
Let’s look at some Lattice Energy data and see what we can learn from
it.
First, let’s look at how lattice
energy changes down a group of the periodic table
Let’s keep the anion the same and change
the cation as in this data for the Group 1 chlorides and the Group 2
chlorides:
Group 1 chlorides Group
2 Chlorides
LiCl 848 BeCl2 3020
NaCl 780 MgCl2 2526
KCl 711 CaCl2 2258
RbCl 685 SrCl2 2156
CsCl 661 BaCl2 2056
Two trends stand out:
First, the Lattice Energy for the
chlorides within each group decreases as the cation size, as measured by its
ionic radius, increases.
This decrease occurs, even though the structure of these chlorides
changes, as we go down the group e.g. sodium chloride (NaCl) has a face centred
cubic lattice with 6:6 coordination.
Whereas caesium chloride (CsCl) has a body
centred cubic lattice structure with 8:8 coordination.
So despite the structure of the halides changing it is the change in ionic radii of the cations that is significantly affecting the
experimentally determined lattice energy.
The change is likely to give rise to a greater distance between ions.
Furthermore the charge density of
the cation is decreasing and reducing the attractive force between cation
and anion.
The polarising effect of the
cation on the softer anion is not that significant since the difference
between the experimentally determined lattice energy and the corresponding
theoretical value is very small.
We also note a second trend
that is shown from this data.
The charge on a Group 1 metal ion is 1+ and on Group 2 metal ions 2+ and
this accounts for the much larger values
for the Group 2 chloride lattice energies.
Let’s look now at changes to the
lattice energy of the halides of Group 1:
Lattice energies of the Sodium Halides (kJ.mol-1)
NaF 918
NaCl 780
NaBr 742
NaI 705
Again we observe that the lattice
energy falls as we go down the group.
The cation radius remains constant but the anion increases in size.
The anion’s charge density is
decreasing as each has the same charge ‘spread’ over a greater surface
area.
The effect is to reduce the force of attraction between ions of opposite
charge.
The other effect that can be seen is that the Iodide has the largest
difference between experimental and theoretical lattice energies.
As predicted the softest lowest
charge density ion, in this case Iodide I-, is the most polarisable by
a small highly charge dense cation Na+.
There is a 28kJmol-1 difference between the two values for the
iodide’s lattice energy.
Here is a typical question from a recent British Advanced level
examination paper covering some of the ideas discussed in these blogs on lattice
energy (they call lattice energy the lattice enthalpy of dissociation.)
1 (a) Define the term lattice enthalpy of
dissociation.
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(2 marks)
1 (b) Lattice enthalpy can be calculated theoretically using a perfect
ionic model. Explain the meaning of the term perfect ionic model.
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1(c) Suggest two properties of ions that influence the value of
a lattice enthalpy calculated using a perfect ionic model.
Property 1 ..........................................................................................................................
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Property 2 ..........................................................................................................................
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1 (d)
Use the data in the table below to calculate a value for the lattice
enthalpy of dissociation for silver chloride.
Enthalpy of atomisation for silver
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+289
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First ionisation energy for silver
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+732
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Enthalpy of atomisation for chlorine
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+121
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Electron affinity for chlorine
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– 364
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Enthalpy of formation for silver chloride
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–127
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(3 marks)
1 (e) Predict whether the magnitude of the lattice enthalpy of
dissociation that you have calculated in part (d) will be less than,
equal to or greater than the value that is obtained from a perfect ionic model.
Explain your answer.
Prediction compared with ionic model
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Explanation .
The ionic radius of Ag+ is 0.065nm and of the chloride ion Cl- 0.180nm.
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